The present invention relates to a hydraulic control apparatus for use in a hydraulically actuated transmission, such as a stepless transmission for vehicles. More particularly, the present invention is directed to a reduction in the number of, for example, hydraulic sensors which is achieved by conducting a feedback pressure control on part of a plurality of pressure controlling valves for adjusting the oil pressure in a hydraulic circuit for a transmission to a predetermined value and by conducting a feedforward pressure control on the other pressure controlling valves.
Japanese Patent Application Laid open (Kokai) No. 2-3751 discloses a speed-change control apparatus for use in a V-belt type stepless transmission for vehicles which is a hydraulic control apparatus for a hydraulically actuated transmission designed to transmit the output of an engine obtained through a fluid coupling to the wheels at a predetermined speed reduction rate. In this hydraulic control apparatus, in order to increase the durability of the belt and thereby stabilize the torque transmission, servo control is conducted on a force applied to the V belt, i.e., an oil pressure to be applied to the input side pulley (primary pulley) and an oil pressure to be applied to the output side pulley (secondary pulley) in accordance with the output of the engine and the torque ratio of the two pulleys. In the stepless transmission, the life time of the V-belt is the important factor, and this servo control conducted on the oil pressures can achieve an increase in the durability of the belt and stabilization of the torque transmission.
In a transmission, such as a stepless transmission, which requires the aforementioned accurate hydraulic control, feedback control is conducted on the line pressure using a hydraulic sensor. Therefore, in the case of a hydraulic circuit having a plurality of oil pressure controlling valves, a hydraulic sensor is provided for each of the oil pressure controlling valves, and feedback control is performed separately for each valve using the sensor output.
However, such a hydraulic sensor is expensive, and hinders cost reduction.